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Subject[RFC PATCH 20/37] mm: implement and enable speculative fault handling in handle_pte_fault()
Date
In handle_pte_fault(), allow speculative execution to proceed.

Use pte_spinlock() to validate the mmap sequence count when locking
the page table.

If speculative execution proceeds through do_wp_page(), ensure that we
end up in the wp_page_reuse() or wp_page_copy() paths, rather than
wp_pfn_shared() or wp_page_shared() (both unreachable as we only
handle anon vmas so far) or handle_userfault() (needs an explicit
abort to handle non-speculatively).

Signed-off-by: Michel Lespinasse <michel@lespinasse.org>
---
mm/memory.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 547d9d0ee962..fc555fae0844 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3103,6 +3103,7 @@ static vm_fault_t wp_pfn_shared(struct vm_fault *vmf)
{
struct vm_area_struct *vma = vmf->vma;

+ VM_BUG_ON(vmf->flags & FAULT_FLAG_SPECULATIVE);
if (vma->vm_ops && vma->vm_ops->pfn_mkwrite) {
vm_fault_t ret;

@@ -3123,6 +3124,8 @@ static vm_fault_t wp_page_shared(struct vm_fault *vmf)
struct vm_area_struct *vma = vmf->vma;
vm_fault_t ret = VM_FAULT_WRITE;

+ VM_BUG_ON(vmf->flags & FAULT_FLAG_SPECULATIVE);
+
get_page(vmf->page);

if (vma->vm_ops && vma->vm_ops->page_mkwrite) {
@@ -3176,6 +3179,8 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)

if (userfaultfd_pte_wp(vma, *vmf->pte)) {
pte_unmap_unlock(vmf->pte, vmf->ptl);
+ if (vmf->flags & FAULT_FLAG_SPECULATIVE)
+ return VM_FAULT_RETRY;
return handle_userfault(vmf, VM_UFFD_WP);
}

@@ -4366,13 +4371,8 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
if (pte_protnone(vmf->orig_pte) && vma_is_accessible(vmf->vma))
return do_numa_page(vmf);

- if (vmf->flags & FAULT_FLAG_SPECULATIVE) {
- pte_unmap(vmf->pte);
+ if (!pte_spinlock(vmf))
return VM_FAULT_RETRY;
- }
-
- vmf->ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd);
- spin_lock(vmf->ptl);
entry = vmf->orig_pte;
if (unlikely(!pte_same(*vmf->pte, entry))) {
update_mmu_tlb(vmf->vma, vmf->address, vmf->pte);
--
2.20.1
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